1. Field of the Invention
The present invention relates to a transmissive diffraction grating, and to a spectral separation element and a spectroscope using the same.
2. Description of Related Art
Spectroscopes, which separate light of different wavelengths, are used widely in measuring equipment, including spectrometers, as well as in reading and writing heads of optical disk recording/reproducing devices, in optical communications, etc. In addition, various diffraction gratings are used as spectral separation elements in such spectroscopes.
Diffraction gratings used for spectroscopy preferably are characterized by a significant wavelength-dependent angular dispersion and a diffraction efficiency (the ratio of the intensities of diffracted light of a specific order and incident light) that is near 100%. Moreover, in applications where the direction of polarization of incident light is not specified, they are also required to have a small difference in diffraction efficiency between TE-polarized light and TM-polarized light, i.e. a small polarization dependent loss (PDL).
A diffraction grating with a multilayer structure, such as the one shown in FIG. 36, has been proposed as a diffraction grating satisfying the above-described conditions and having a structure that is easy to fabricate (for example, see Manabu Shiozaki, Masaichi Mobara “Polarization-independent design of multilayered diffraction gratings having large angular dispersion”, SEI Technical Review, September 2004, No. 165, pp. 38-42). As shown in FIG. 36, in a diffraction grating 110, a glass substrate is used as a substrate 10 with a low refractive index, and ridges 21 including a high refractive index layer 12 and a low refractive index layer 13 are provided on the substrate 10. When light 50 is incident upon the diffraction grating 110 from the direction of the ridges 21 at a predetermined angle of incidence θ, normal to longitudinal direction of the ridges 21, first-order diffracted light 51 passes through, and is emitted from, the substrate 10. In such a case, providing the low refractive index layer 13 reduces reflectance, and, as a result, improves the efficiency of diffraction of the first-order diffracted light 51.
The diffraction grating 110 shown in FIG. 36 is commonly fabricated according to the following processes (1) to (4).
(1) First of all, the material of the high refractive index layer 12 and material of the low refractive index layer 13 are deposited successively on the substrate 10.
(2) Next, a mask pattern is formed on the surface of the material of the low refractive index layer 13.
(3) Then, portions other than the mask pattern are etched away.
(4) Finally, the mask pattern is removed.
However, if the depth of etching is insufficient during the fabrication of the diffraction grating 110 of FIG. 36, then, as shown in FIG. 37, the high refractive index layer 12 remains at the bottom of the grooves 30. The high refractive index layer 12 remaining at the bottom of the grooves 30 causes a significant deterioration in the characteristics of a diffraction grating 120. Accordingly, it was necessary to control the depth of etching stringently when fabricating the diffraction grating 110 of FIG. 36.
Moreover, while it is more advantageous to have a larger number of design parameters in order to improve diffraction efficiency and reduce PDL, in case of the diffraction grating 110 of FIG. 36, the relatively freely variable design parameters include three parameters, such as the thickness of the high refractive index layer 12, the thickness of the low refractive index layer 13, and the width W of the ridges 21, which cannot be considered a sufficient number of design parameters for improving diffraction efficiency and reducing PDL. Additionally, because refractive indices depend on the material used, it is difficult to freely select their values.
Accordingly, in order to eliminate such problems, a diffraction grating has been proposed that has a three-layer ridge structure including a first layer, a second layer, and a third layer with non-continuous refractive indices (e.g. see International Publication No. WO2004/074888).
Although the shape of the cross-section of the ridges perpendicular to their longitudinal direction in the diffraction grating disclosed in International Publication No. WO2004/074888 is rectangular (with a constant width), in some cases it is technically difficult to keep the width of the ridges constant. In other words, fabrication of the diffraction grating disclosed in International Publication No. WO2004/074888 is extremely difficult. Also, if the shape of the ridges deviates from the design values, diffraction efficiency, PDL, and other characteristics may deteriorate.